ARTICLE pubs.acs.org/JPCC
Charge-Carrier Dynamics in Nitrogen-Doped TiO2 Powder Studied by Femtosecond Time-Resolved Diffuse Reflectance Spectroscopy Ken-ichi Yamanaka* and Takeshi Morikawa Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan
bS Supporting Information ABSTRACT: Nitrogen-doped TiO2 (N-TiO2) is one of the most promising and widely investigated visible-light-responsive semiconductor photocatalysts. Femtosecond time-resolved diffuse reflectance (TDR) spectroscopy was employed under weak excitation conditions to clarify the charge-separation and trapping dynamics in N-TiO2 powder, which exhibits high activity for photocatalytic oxidation under visible light. TDR spectra of N-TiO2 after 360 nm light excitation (Ti 3d r O 2p transition) revealed that the surface-trapped electrons and holes were generated immediately after excitation, similar to that for TiO2, and the population of surface-trapped electrons decreased more than that for TiO2 due to deep trapping by additionally induced oxygen vacancies. The dependence of trapping dynamics on the N concentration is discussed with respect to the diffusion coefficient of electrons. TDR spectra of N-TiO2 after 450 nm light excitation (Ti 3d r N 2p transition) clearly indicated the generation of charge carriers. Compared with the 360 nm excitation, two differences of time evolution were detected: the significant decrease just after excitation and the deep trapping of electrons within 1 ps (time resolution). Possible mechanisms for the dependence of the charge-carrier dynamics in N-TiO2 on the excitation wavelength are discussed.
’ INTRODUCTION TiO2 is the most widely investigated photocatalyst and is used in various practical applications.1 However, only a small UV fraction of solar light can be utilized by TiO2 because of the large band gap of TiO2 photocatalysts. To improve the utilization of solar energy, the development of TiO2 photocatalysts with extended absorption into the visible wavelength region is required. Nitrogen-doped TiO2 (N-TiO2) is one of the most promising and widely investigated TiO2 photocatalyst systems.2 The photocatalytic activity of N-TiO2 powder under visible light irradiation was revealed by Asahi et al., and N-TiO2 powder maintained similar photocatalytic activity under UV light irradiation to that of TiO2 powder before N-doping.2b This excellent photocatalytic activity was obtained at a reasonably low cost, so that N-TiO2 has already been put to practical use on the market.3 The mechanism of visible light response has attracted much interest and has been studied using such techniques as quantum mechanical calculation,2b,4 X-ray photoelectron spectroscopy (XPS),5 electron paramagnetic resonance (EPR),6 deep level optical spectroscopy (DLOS),7 and photoelectrochemical measurements.2h Livraghi et al. revealed that the single atom nitrogen centers in bulk TiO2 play an essential role in the absorption of visible light, in the promotion of electrons to the conduction band (CB), and in photoinduced electron transfer to reducible adsorbates.6 The dynamics of charge carriers in photocatalysts is also important to gain an understanding of the photocatalytic reaction. To this end, transient absorption spectroscopy has been one r 2011 American Chemical Society
of the most powerful tools employed. Transient absorption studies of TiO2 have revealed initial trapping processes of photoelectrons and photoholes8 and the oxidation and reduction process of adsorbed molecules.9 However, to the best of our knowledge, there have been only a few reports on the charge carrier dynamics in N-TiO2. Tachikawa et al. investigated the photocatalytic degradation process of ethylene glycol by pure anatase TiO2 and N-TiO2 powders under UV or visible light irradiation using nanosecond time-resolved diffuse reflectance spectroscopy.10 The scavenging of photogenerated holes by ethylene glycol occurred during 355 nm laser photolysis, and no direct oxidation reaction of ethylene glycol occurred during 460 nm laser photolysis in acetonitrile, although sufficient generation of charge carriers occurred upon excitation.10 Katoh et al. studied charge separation and trapping processes in N-TiO2 photocatalysts using time-resolved microwave conductivity (TRMC),11 which revealed that the trapping rate increased with N-doping due to an increase in the oxygen vacancies, and the charge separation efficiency under visible light excitation was found to be one-third of that under UV excitation.11 Tang et al. studied the dynamics of the photoelectrons and photoholes using transient absorption spectroscopy in order to understand the lack of activity of N-TiO2 film for photocatalytic water oxidation under visible light.12 They confirmed that O2 production from Received: September 23, 2011 Revised: November 16, 2011 Published: November 22, 2011 1286
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water over N-TiO2 is a slow process and reported that the lack of water oxidation was due to electronhole recombination between the charge carriers trapped at doping-induced states. However, the primary process of charge carriers in N-TiO2 in the femtosecond to picosecond time region remains unclear, despite their importance in elucidating the difference of charge separation between UV and visible excitation and the enhancement of carrier trapping processes by N-doping-induced oxygen vacancies. In this paper, we report the charge-carrier dynamics in N-TiO2 powder studied using femtosecond time-resolved diffuse reflectance spectroscopy. The enhancement of carrier trapping processes by N-doping-induced oxygen vacancies and the difference in the carrier dynamics between UV and visible excitation are discussed.
’ EXPERIMENTAL SECTION Materials. N-TiO2 powder was prepared by treating anatase TiO2 powder (ST01, Ishihara Sangyo Kaisha) in a NH3 (67%)/ Ar atmosphere at 550, 600, and 650 °C for 3 h followed by annealing at 300 °C for 2 h in flowing air.2b,p The nitrogen concentrations estimated by XPS were 0.02, 0.25, and 0.43 atom % for the powder treated at 550, 600, and 650 °C, respectively. All N-TiO2 samples exhibited typical anatase X-ray diffraction (XRD) peaks, and no peaks corresponding to rutile or brookite phases were detected. Methanol-d4 (CD3OD, Aldrich, 99.8 atom % D) was used without further purification. Steady-State Spectroscopy. Absorption spectra were measured using a spectrophotometer (Shimadzu, UV-3600) at room temperature (RT) as diffuse reflectance spectra. The diffuse reflectance spectra were estimated using the KubelkaMunk function:
K=S ¼ ð1 rÞ2 =2r
ð1Þ
where K and S are the absorption and scattering coefficients, respectively, and r is the diffuse reflectance. Here, a relative value of r was measured using a diffuse white standard (Spectralon, Labsphere, Inc.) as a reference. The change in the absorbance of N-TiO2 powder with adsorbed CD3OD was measured after UV light irradiation (270380 nm) for 1 h. CD3OD was adsorbed onto the surface of N-TiO2 powder by vacuum drying of the suspensions. A Xe lamp (Hamamatsu, 150 W) was used as an excitation light source; the light beam was passed through a UVtransmittingvisible-absorbing filter (Hoya, U340) and a heatabsorbing filter (Hoya, HA50) prior to illumination of the sample. Time-Resolved Diffuse Reflectance Spectroscopy. Transient absorption spectra of N-TiO2 powder were obtained using a femtosecond time-resolved diffuse reflectance instrument (described elsewhere).13 Briefly, the output of a mode-locked Ti: sapphire oscillator (Coherent, Vitesse), which was pumped by the second harmonic generation (SHG) of a continuous wave Nd3+:YVO4 laser (Coherent, Verdi), was amplified with a regenerative amplifier (Coherent, Legend), which was pumped by a Nd3+:YLF laser (Coherent, Evolution). The output of the amplifier (2.4 W, 100 fs fwhm, 1 kHz) was divided into two parts. One part was converted to the fourth harmonic generation (FHG, 360 and 390 nm) of signal (1440 and 1560 nm) or the FHG (420 and 450 nm) of idler (1680 and 1800 nm) using an optical parametric amplifier (OPA) system (Coherent, OPerA). The OPA output was used as a pump light after the repetition
Figure 1. (a) KubelkaMunk functions for TiO2 and N-TiO2 powders. (b) Difference spectrum of N-TiO2 powder (0.25 atom %) with adsorbed CD3OD after UV light excitation in an Ar atmosphere at RT.
rate was modulated to 0.5 kHz using a chopper. The other part of the amplified Ti:sapphire output was led through an optical delay circuit with a computer-controlled stepping motor and then focused into a 5 mm H2O cell to generate a white-light continuum. The white light was collimated using an achromatic lens and sent through a filter (Hoya, CAW500) for elimination of the fundamental light. A small portion of the collimated white light was used as the reference light and the residual light was focused on the sample. Sample powder was filled into a 2 mm thick cell made of specially selected luminescence-free quartz plates (Daico MFG, USQ-E24). The diffuse reflected light from a sample was collected by a lens and detected using a combination of a polychromator (Solar TII, MS3501I) and a multichannel photodiode array detector (CDP, ExciPro). The transient absorption intensity of the TDR spectrum is presented as percentage absorption, %Abs = (1 R/R0) 100, where R and R0 represent the intensity of the diffuse reflected light of the probe pulse, with and without excitation, respectively.14 The time resolution of this system was less than 1 ps. Hereafter, TDR spectra is given the same meaning as transient absorption spectra, because TDR spectra correspond directly to the transient absorption spectra and the linearity between %Abs and the concentration of transient species can be satisfied within the experimental error when %Abs is below 15%.15 The observed decay curves were analyzed using commercially available software (Wave Metrics, IGOR Pro).
’ RESULTS AND DISCUSSION Steady-State Absorption Spectra. Figure 1 shows absorption spectra for the TiO2 and N-TiO2 powders. The spectra were estimated using the KubelkaMunk function (eq 1). TiO2 powder only had an absorption in the UV spectral region, because of the band gap of 3.2 eV for the anatase phase. For N-TiO2, the absorption intensity in the visible spectral region below 500 nm (400500 nm) increased with the amount of N atoms doped in the TiO2. In particular, for N-TiO2 doped with 1287
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Figure 2. Time profiles of the transient absorption for N-TiO2 (0.25 atom %) powder at 660 nm after 360 nm excitation. The signals were normalized at the maxima.
0.43 atom % N, a new absorption band at a wavelength greater than 500 nm was observed. Absorptions below 500 nm were mainly due to nitrogen states located above the valence bands (VB), and the broad absorption above 500 nm was probably caused by oxygen vacancies.16 In this study, the discussion centers around N-TiO2 (0.25 atom %) powder, because it exhibits the highest activity for photocatalytic oxidation under visible light.2b,p Figure 1b shows the change in the absorbance of N-TiO2 (0.25 atom %) powder with adsorbed CD3OD after UV light irradiation (270380 nm) for 1 h in an Ar atmosphere. Photoinduced holes are quenched by the photocatalytic oxidation of CD3OD, so that the remaining absorption spectrum was attributed to photoinduced electrons in N-TiO2.17 A shoulder around 600 nm and a monotonic increase in absorption above 700 nm were detected. The spectral shape is consistent with anatase TiO2 film according to the literature;17 therefore, the former and latter bands were assigned as electrons trapped at the surface and free or shallow-trapped electrons in the bulk, respectively. The center position of the absorption shoulder (∼600 nm) was shifted several tens of nanometers compared to the anatase TiO2 film,17 which suggests that the electrons in the surface of N-TiO2 were trapped by relatively deeper defect sites. The absorption due to the bulk electrons was analyzed according to a theoretical model for the optical transition of intraband and shallow-trapped charge carriers,18 in which the absorbance is proportional to λn. In an ideal case, n assumes a value of 3/2, 5/2, and 7/2 for the scattering of acoustic phonons, optical phonons, and ionized impurities, respectively.18c The experimental results can be explained by n = 1.3 for N-TiO2 (0.25 atom %) powder, which indicates that the free carrier absorption occurs mainly by the acoustic phonon scattering mechanism, as is the case for anatase TiO2 film (n = 1.7). TDR Spectra after 360 nm Light Excitation. Time-resolved measurements under weak excitation conditions are required to investigate the charge separation and trapping dynamics, because second-order electronhole recombination can be neglected when the number of photoinduced electronhole pairs is less than unity in a single TiO2 or N-TiO2 particle.8f,17 Therefore, the dependence of the TDR on the excitation light intensity was examined first. Figure 2 shows normalized time profiles of TDR spectra for N-TiO2 (0.25 atom %) at 600 nm after 360 nm excitation. Although the decay slows as the excitation intensity is decreased from 190 to 20 μJ cm2, the TDR intensity is unchanged below 20 μJ cm2, which indicates that second-order bulk recombination can be ignored by excitation below 20 μJ cm2 at 360 nm.
Figure 3. TDR spectra for (a) TiO2 and (b) N-TiO2 (0.25 atom %) powders after 360 nm laser excitation (15 μJ cm2).
Figure 3a shows TDR spectra of anatase TiO2 powder obtained under weak excitation conditions (360 nm, 15 μJ cm2). To detect the weak TDR signal, the spectral data at every fixed delay were averaged over 4000 pulse shots. At 1 ps, a broad absorption band was observed, which decreased slightly above 550 nm until 1600 ps, whereas the absorption below 550 nm remained unchanged. This time evolution is consistent with that of the anatase TiO2 film, in which the surface-trapped holes (around 500 nm) and the surfacetrapped electrons (around 800 nm) were generated immediately after excitation, followed by a gradual decay of the surface-trapped electron population over 1 ns, although the population of surfacetrapped holes remained constant.8f This agreement of the present results with those previously reported for anatase TiO2 films also indicates the accuracy of the present TDR measurements. TDR spectra of N-TiO2 (0.25 atom %) measured in the same manner as that for the TiO2 powder are presented in Figure 3b. At 1 ps, a broad absorption band appeared, similar to TiO2 powder, which suggests the generation of surface-trapped holes and electrons as for TiO2 powder. The 360 nm laser excitation resulted in Ti 3d r O 2p transition; therefore, it is reasonable to consider that the effect of nitrogen doping was less within 1 ps. The intensity of the 550800 nm band then decreased faster than that for TiO2 powder, whereas the 400500 nm band remained unchanged. At 1600 ps, the peak position was shifted to around 550 nm, which is approximately 50 nm shorter than that for TiO2 powder. According to previous reports,8f,17 the difference of time evolution in the subnanosecond time domain suggests that the decrease in the population of surface-trapped electrons is promoted by nitrogen doping. It has been reported that nitrogen doping introduces oxygen vacancies, according to both experiments and calculations.2b,4,6,7 Generally, two N atoms can reasonably substitute three O atoms in TiO2, assuming the valence state of the Ti atom remains at Ti4+, which results in the formation of one oxygen vacancy. According to the DLOS results for N-TiO2 film, the energy levels of oxygen vacancies are located at 1.18 eV below the CB, and the 1.18 eV level is potentially an efficient generationrecombination center, because the 1.18 eV level strongly interacts with both the conduction and VBs.7 1288
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Recently, Katoh et al. reported that the trapping rate of electrons increased with N-doping due to the increase in oxygen vacancies observed from TRMC measurements of N-TiO2 powder in the nanosecond time region.11 Therefore, the present TDR results for N-TiO2 powder can be explained by the deep trapping of electrons by the additionally induced oxygen vacancies. The time dependence of the TDR signal due to surfacetrapped electrons was examined quantitatively. Figure 4 shows time profiles for TiO2 powder and three different N-TiO2 powders around 660 nm. The signals were averaged over 21 channels (655665 nm). All signal decreased until around 1 ns and then the intensity became almost constant. The time dependence was analyzed by a least-squares fitting procedure using the following function, assuming a long-lived component as a constant: %AbsðtÞ ¼ A expð t=τÞ þ C
ð2Þ
where τ is the lifetime and A and C are fitting parameters, respectively. The fitting results are summarized in Table 1. For TiO2 powder, τ, A, and C were estimated to be 320 ( 90 ps, 24%, and 76%, respectively. In the case of N-TiO2 (0.25 atom %), τ was 0.9 times smaller and A was 1.6 times larger than that for TiO2 powder. C was 1.2 times smaller, which was consistent with the TRMC results. The TRMC signal for N-TiO2 (0.25 atom %) powder just after nanosecond laser excitation (355 nm) was 1.4 times smaller than that for TiO2 powder.11 The decrease in τ was nonlinear with increasing concentration of N atoms, although A was almost constant (40%) for all N-TiO2 concentrations (Table 1). We now discuss the mechanism of this complicated dependence on the N concentration. The number of surface-trapped electrons generated by 360 nm laser excitation (Ti 3d r O 2p transition) is almost the same for each N-TiO2 powder. The number of oxygen vacancies that trap electrons increases with increased nitrogen doping; therefore, the decay should be accelerated. However, τ for the N-TiO2 (0.25 atom %) powder was 1.5 times smaller than that for the N-TiO2
Figure 4. Time profiles for TiO2 and N-TiO2 powders at 660 nm. Solid lines indicate fitting results.
(0.02 atom %) powder, and the decrease of τ for N-TiO2 (0.43 atom %) to that for N-TiO2 (0.25 atom %) was only slight. Therefore, a possible mechanism is proposed in which the electron trapping process is diffusion-limited. The diffusion coefficients, D, of free electrons and electrons before deep trapping in the CB of TiO2 are 1.0 106 and 2 107 m2 s1, respectively.19 Assuming that each N-TiO2 composition has the same D value of 2 107 m2 s1 with τ = 300 ps, then the diffusion length, L, is estimated to be 8 nm from following equation: L ¼ ðDτÞ1=2
ð3Þ
The value of 8 nm is close to the radius of the N-TiO2 powder (19 nm diameter). On the other hand, the number of oxygen vacancies is probably large enough, even in N-TiO2 (0.02 atom %), that all surface-trapped electrons would be quenched within several nanoseconds, which would result in no dependence of A and C on the N concentration. Another possible mechanism is the trap-filling effect, where generated electrons are immediately trapped and occupy deep traps in the particles, which would result in an increase of the effective mobility of other electrons.20 However, the number of photoinduced electronhole pairs is less than unity in a single N-TiO2 particle according to the excitation below 20 μJ cm2 at 360 nm (Figure 2); therefore, this trap-filling effect is a minor process in this system. In addition, the time profiles at 760 nm were also analyzed in the same manner. The dependence of N-concentration on fitting parameters shows the same tendency of that obtained from the
Figure 5. TDR spectra for (a) TiO2 and (b) N-TiO2 (0.25 atom %) powders after 450 nm laser excitation (150 μJ cm2).
Table 1. Fitting Parameters (τ, A, and C) for eq 1 sample
τ/ps
A
C
TiO2 N-TiO2 (0.02 atom %)
320 ( 90 420 ( 80
0.16 ( 0.02 (24 ( 2%) 0.31 ( 0.02 (40 ( 3%)
0.52 ( 0.02 (76 ( 2%) 0.47 ( 0.02 (60 ( 3%)
N-TiO2 (0.25 atom %)
280 ( 30
0.35 ( 0.01 (38 ( 1%)
0.58 ( 0.01 (62 ( 1%)
N-TiO2 (0.43 atom %)
260 ( 20
0.34 ( 0.01 (41 ( 1%)
0.49 ( 0.01 (59 ( 1%)
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Figure 6. Time profiles of TDR spectra for TiO2 (dashed lines) and N-TiO2 (0.25 atom %) powders (solid lines) at 660 nm after laser excitation at 360 nm (15 μJ cm2), 390 nm (35 μJ cm2), 420 nm (120 μJ cm2), and 450 nm (150 μJ cm2). Inset shows the short time region.
time profiles at 660 nm (see Supporting Information, Figure S2 and Table S1). TDR Spectra after Visible Light Excitation. Figure 5a shows TDR spectra for TiO2 powder after 450 nm excitation. A broad band centered around 600 nm appeared at 0.4 ps after laser irradiation, although it is impossible for TiO2 powder to efficiently absorb 450 nm light. The broad band decreased quickly and the intensity of the TDR signal became less than 0.4% at 5 ps. The intensity of a small portion of the absorption decreased gradually until 1600 ps. Figure 6 shows the time profiles at 660 nm (dashed lines) together with the results for excitation at 360, 390, and 420 nm. A fast decay, within 5 ps, was also observed, except for 360 nm excitation. The value of the signal at 1600 ps was estimated to be 0.3 for 390 nm excitation, while almost all the signal decreased for excitation at 420 and 450 nm. The absence of the TDR signal by excitation at >420 nm is reasonable, due to the band gap of anatase TiO2 (3.2 eV). The fast decay in the TDR spectra of TiO2 powder just after excitation has been argued by several groups.21 Furube et al. investigated the recombination dynamics of photogenerated charge carriers generated in several TiO2 photocatalytic powders by 390 nm light.21a They showed that the electronhole recombination kinetics were dependent on the crystal structure of the TiO2 powders and that the charge carriers in anatase TiO2 recombine faster than those in rutile TiO2.21a On the other hand, Noguchi et al. investigated the dependence of the TDR signals on the excitation wavelength for anatase and rutile TiO2 powders21b and reported that a rapid decay was observed for both anatase and rutile TiO2 powders when TiO2 powder was excited by light pulse of near-band-gap energy, although the origin of the rapid decay has remained unclear.21b Our experimental conditions are close to those employed by Noguchi et al. and we also obtained the same TDR spectra for rutile TiO2 powder, in which the fast decay disappears upon excitation at 390 nm and appears upon excitation at >420 nm (see Supporting Information, Figure S3). Some possible mechanisms for the fast decay will be discussed later. The small absorption followed by the initial decrease was also observed for TiO2 powder after 410 nm excitation.21b The possibility of a two-photon absorption process was indicated by Noguchi et al.21b Figure 5b shows TDR spectra for N-TiO2 (0.25 atom %) powder. Although the intensity of excitation laser pulse at 450 nm (150 μJ cm2) is 10 times larger than that at 360 nm (15 μJ cm2),
Figure 7. Comparison of TDR spectra for N-TiO2 (0.25 atom %) powder at 5 ps after 450 nm excitation with (a) that at 1 and 1600 ps after 360 nm excitation and (b) that at 0.4 ps after 450 nm excitation.
the number of photoinduced electronhole pairs in a single N-TiO2 particle remained below unity, because the absorption coefficient decrease was more than 10-fold at 450 nm. The broad band centered around 600 nm was also observed. The peak intensity of TDR signal was 2.4%, which is similar to that for TiO2 powder, although the absorption coefficient of N-TiO2 (0.25 atom %) at 450 nm differs significantly from that of TiO2 powder. At 5 ps, the intensity also decreased quickly, but approximately 20% of the peak intensity remained, after which no decrease was observed until 1600 ns. Figure 6 shows the time profiles at 660 nm (solid lines). The fast decay within 5 ps was also observed by >390 nm laser excitation, but more than 0.3% of the signal remained until 1600 ps, whereas almost all the signal was decreased in the case of TiO2 powder excited at 420 and 450 nm. This result clearly indicates the presence of charge carriers in N-TiO2 powder after visible light irradiation (Ti 3d r N 2p transition). In comparison with the 360 nm excitation, the differences in the time evolution after 450 nm irradiation were the significant decrease just after excitation and the lack of decrease after 5 ps. The former phenomenon is discussed now and the latter is addressed in the next paragraph. The peak intensity of the TDR signal for N-TiO2 powder was almost the same as that for TiO2 powder after visible light excitation, despite the significant difference in their absorption coefficients. Thus, it is highly unlikely that the rapid decay can be attributed to some photophysical or photochemical process, such as surface trapping or charge recombination. A plausible mechanism is proposed by Tamaki et al.; an experimental artifact due to simultaneous twophoton absorption involving a single pump photon and a single probe photon, which was observed for TiO2 film.8f This mechanism consistently explains our experimental results. The fast decay appeared only when the excitation pulse diffuses in TiO2 or N-TiO2 powders. The 360 nm light was fully absorbed by TiO2 or N-TiO2 powder, so that no fast decay appeared. To consider the reason for the lack of decrease in the TDR signal after 5 ps obtained by 450 nm excitation, the spectral shape at 5 ps was compared with that for 360 nm excitation, as shown 1290
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Figure 8. Schematic illustration of the spatial and energetic distribution of electrons and holes in N-TiO2 powder after excitation at (a) 360 and (b) for 450 nm. VO indicates an oxygen vacancy.
in Figure 7a. These spectra are normalized at 500 nm. The spectral shape at 5 ps after 450 nm excitation is similar to that at 1600 ps after 360 nm excitation, although the intensity around 600 nm is slightly larger. The spectral shape at 5 ps after 450 nm excitation is unchanged from that at 0.4 ps (Figure 7b), which suggests that the electrons are trapped deeply within 1 ps (time resolution). This ultrafast trapping process may be due to oxygen vacancies induced near N atoms. Lee et al. investigated the surface doping states and energy band gap properties by XPS and density-functional theory (DFT) applied to a 2 2 1 supercell of N-TiO2 with oxygen vacancies.22 When one O site in the supercell was replaced by a N atom and also one O site was removed to introduce an oxygen vacancy, the electronic band structure corresponded to that for the present N-TiO2 powder. In this model, the distance between the N atom and oxygen vacancy is approximately 5 Å. The distribution of excess charge created by a single O atom vacancy on a rutile TiO2(110) surface was investigated by Minato et al.23 Scanning tunneling microscopy (STM) and DFT show the charge to be delocalized over multiple surrounding titanium atoms, contrary to the conventional model, where the charge remains localized at the defect. The photoexcited electron is considered to be distributed at the Ti 3d level around the N atom just after the Ti 3d r N 2p transition by 450 nm excitation, so that the electron in the Ti 3d level probably transits quickly to the defect level formed by the adjacent oxygen vacancy. The excitation wavelength dependence of trapped electrons in N-TiO2 powder was determined using nanosecond TRMC measurements.11 Katoh et al. reported that the charge separation efficiency under 450 nm light excitation was one-third as high as
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the efficiency observed under 355 nm light excitation.11 The observation of a considerable decay of more than 60% of the peak intensity due to a decrease in the electron population was expected in the picosecond time region; however, the intensity of the TDR signal remained almost constant after an initial decrease. The possibility of the initial decrease being attributed to a charge recombination can be denied because of its appearance in the TDR results for TiO2 after 450 nm excitation. Therefore, the reason for the small charge separation efficiency obtained by TRMC remains unclear at the present stage. The trapping dynamics of electrons and holes in N-TiO2 powder is summarized in Figure 8. Two energy levels for the oxygen vacancies (0.75 and 1.18 eV below CB minimum) are reported.6,24 It has been experimentally confirmed that the DLOS signal at 1.18 eV becomes strong with an increase in the N concentration for N-TiO2 film.7 This DLOS study also revealed the N atom level (2.48 eV).7 A shallow defect level and a surface defect level are also illustrated qualitatively. For 360 nm excitation (Figure 8a), free electrons and holes are generated in the CB and VB, respectively. Transient absorption spectra for N-TiO2 were almost the same as those for TiO2. According to previous studies on TiO2 film,8f,19b both free electrons and holes are trapped rapidly at surface trap states (
